1. Field of the Invention
The present invention concerns shrunken moldable beads of foamed thermoplastic polymers, e.g., polyolefins, and particularly beads of crosslinked polyolefins.
2. Description of the Prior Art
Foamable polystyrene beads are relatively easy to obtain and mold. Polystyrene resin is impregnated with an expanding agent, usually pentane, either during polymerization or resin particles are impregnated after polymerization. These particles are then subjected to steam to partially expand them. The pre-expanded beads are then fed to a mold and subjected to pressurized steam where they further expand, fuse together, and conform to the shape of the mold. Such moldings are useful as decoration, insulation, and protective packaging; however, expanded polystyrene moldings suffer from many disadvantages. Polystyrene exhibits poor solvent resistance and high temperature stability and, therefore, moldings made from polystyrene cannot be used for many applications. Expanded polystyrene foam is generally brittle, fragile and possesses poor cushioning properties which limits its use as protective packaging for fragile items such as computers and other delicate instrumentation. In addition, polystyrene foam does not stand up well to repeated impacts; usually the cushioning ability of the molding is severely impaired after just one impact.
Foams molded from polyolefin beads overcome many of the drawbacks of polystyrene foam. Generally available polyolefin foam beads are polypropylene or crosslinked polyethylene. Both of these materials possess greater solvent resistance than polystyrene and are also more resistant to high temperature. Polyolefin foam is much more resilient and flexible than polystyrene foam and, therefore, is of much greater use for the packaging of fragile items. It maintains much of its cushioning effect after even many impacts and therefore lends itself for use as packaging for long distance transport or for re-useable packages.
In the case of polyethylene, a substantially crystalline polymer, the temperature range for good molding of foam beads is quite narrow. If the molding temperature is too low, poor fusion will result and the molding will not possess optimum tear resistance and large voids or unfused pockets could exist in the molding. If the molding temperature is too high, the plastic becomes too flowable and the structural integrity of the foam is destroyed, resulting in a collapsed, misshapen molding.
To give the polyethylene a greater resistance to temperature and to widen the temperature range for molding, polyethylene is crosslinked. This allows the foam to be molded using steam as the heat source without being destroyed. Moldable crosslinked polyethylene foam beads are presently manufactured in several ways. Polyethylene beads containing a chemical crosslinking agent, such as dicumyl peroxide, can be suspended in an aqueous solution and heated to the proper temperature to trigger the crosslinking reaction. Polyethylene resin can also be crosslinked by subjecting the particles to high energy radiation, such as X-rays or electron beams.
The resultant crosslinked resin particles can then be impregnated with a hydrocarbon or chlorofluorocarbon blowing agent, such as butane, pentane, dichlorodifluoromethane, etc., by charging an aqueous suspension of the crosslinked polyethylene beads under pressure with the blowing agent. The solution is then heated and stirred in the autoclave to impregnate the beads with the blowing agent. Such processes are described in U.S. Pat. Nos. 4,399,087 and 4,436,840.
Because the blowing agent incorporated in the crosslinked polyethylene particles will readily dissipate, the expandable beads must either be stored under pressure or immediately pre-expanded, which is usually the case. The expansion ratio of these pre-expanded beads is usually between 10 and 45 to 1. Before molding, these beads are usually subjected to a pressurizing step where the beads are placed in a container which is charged with a pressurized gas, usually air or a chlorofluorocarbon/air mixture. Such processes are described in U.S. Pat. Nos. 4,399,087 and 4,443,393. This step raises the pressure of the gas inside the cells of the foam beads above atmospheric pressure to impart additional expandability needed during molding. The beads must be molded soon after this step or the additional pressure inside the cells of the beads will be dissipated.
In another method, low density polyethylene resin and a hydrocarbon or chlorofluorocarbon blowing agent are melt mixed and extruded into strands which are cut into beads. These beads are then exposed to high energy radiation to crosslink the polymer structure and impart the thermal resistance needed to easily mold the particles. These beads require special molding apparatus as no additional expandability is incorporated into the beads prior to molding.
The first (chemical) method of crosslinked polyethylene bead manufacture is disadvantageous in that a relatively large and expensive autoclave-type reactor is needed for blowing agent impregnation. This is also a batch process where a certain quantity of the moldable crosslinked polyethylene beads are manufactured at once and then this entire quantity of beads must be treated and/or stored. This requires large storage facilities. In addition, these beads must be pressure treated prior to molding to impart additional expandability to the foam. This process requires substantial time, as the beads will be destroyed or damaged if the pressurizing step is carried out too quickly. Therefore, large pressure containers are needed to perform this operation economically.
Using the second (radiation) process discussed, the crosslinked beads can be made on a relatively inexpensive extruder equipped with the proper equipment for granulating the foamed extrudate. However, to crosslink the foam, a relatively expensive and cumbersome radiation source is required. Generally, it is not feasible to perform the crosslinking step in a number of manufacturing locations but the process lends itself to one or several rather large, central manufacturing facilities. High energy radiation does not easily or quickly penetrate into the foamed plastic structure. Therefore, the degree of crosslinking can be much less on the inside portions of the foamed beads than on the outsides, which could cause the beads to possess deficient thermal resistance.
U.S. Pat. No. 3,413,244 discloses a process for producing cellular polyolefin products in which a particulate unfoamed polyolefin is foamed within a mold and is simultaneously grafted and crosslinked by units of compounds containing two non-conjugated ethylenically-unsaturated double bonds.
International Application No. PCT/F184/00079, International Publication Number WO 85/01944, discloses foamed, silane-crosslinked polyolefin foam cable coverings which are described as relatively hard and rigid and are produced by extruding a mixture containing polyethylene, a silane hydrolyzable with water, a condensing catalyst and a foaming agent such as water.
U.S. Pat. No. 4,333,898 discloses a method for production of relatively high density foamed polymers (such as polyethylene) in which the polymer is mixed with a silane, which grafts thereto, then extruded to provide a jacket for a cable or the like, with a moist inert gas being injected into the extruder just prior to extrusion to cause the polymer to foam and the silane-grafted polymer to crosslink.
U.S. Pat. No. 4,456,704 discloses a method for producing crosslinked polyethylene foams which comprises mixing a polyolefin resin, a blowing agent, and optionally, a surface active agent, the polyolefin resin containing a crosslinkable ethylene polymer having on the side chains thereof silyl groups which effect crosslinking upon contact with water; extruding the mixture into a low pressure zone where the resulting extrudate, e.g., sheet, is allowed to expand, and bringing the expanded extrudate into contact with a silanol condensing catalyst so that the expanded extrudate is crosslinked upon contact with water.
None of these patents disclose a process for the extrusion of a silane-modified polyolefin containing a silanol condensation catalyst, with a blowing agent being injected to produce moldable shrunken foamed beads which crosslink internally when exposed to moisture.
A wide variety of thermoplastic polymers have been used in the preparation of foams which are moldable into various shapes, either directly or through the use of moldable beads, pellets or the like. See, e.g., U.S. Pat. No. 4,323,528 of Collins, assigned to the predecessor of applicant's present assignee, describing the use of polystyrene, high and low density polyethylene, and polyvinyl chloride.
Pellets of thermoplastic polymeric foams have been produced which do not require the usual pressurization or crushing prior to molding (a process sometimes referred to as "atmospheric molding"). For example, in Ando's U.S. Pat. No. 4,483,809 (assigned to Kanegafushi) small crosslinked polyethylene particles are fed to an autoclave equipped with a stirring device. The particles are dispersed in water with the aid of some dispersants to prevent flocculation of the particles. A blowing agent is added in the proper amount. The temperature and pressure of the autoclave are raised as the dispersion is stirred. It is held at a temperature and pressure for the time required to effect absorption of the blowing agent into the softened polyethylene. The autoclave is then opened and the beads foam as they are ejected. According to the patent, the beads are then subjected to an "expansion ratio adjustment" step to make them moldable without the usual pressurization or crushing step required for molding polyethylene beads. In this step they are heat treated so as to relax the polymer constituting the cell walls so the beads reduce in size. The resulting beads do not appear shrunken but maintain a spherical shape and smooth skin and the cells appear to be fully inflated. During this "shrinkage" process, each cell in the foam could be likened to a balloon which is losing some of its inflatant--it maintains its general shape, but merely becomes smaller. The Kanegafuchi process is essentially a two-step process, first bead manufacturing and then an expansion ratio adjustment step, with the additional factor that a specially formed resin particle is the basic raw material. Such small, round particles are not the form of the usual commercially available polyethylene or polypropylene resins. In the case of low density polyethylene, this resin must be crosslinked, either by radiation or by chemical addition (before particle formation).
See also U.S. Pat. No. 3,766,099, which discloses the production of ethylenic resin foams by using volatile organic foaming agents to form flowable gels which can be extruded to form the foam. According to this patent, with the use of certain foaming agents whose gas permeability through the polymeric membranes forming the cell walls of freshly-extruded foam exceeds the corresponding permeability of air, the cells will soon become collapsed by atmospheric pressure, with the result that uneven wrinkles and hollows form on the surfaces of the foam. The patent discloses that the foaming agent can be selected or blended to adjust its gas permeability through the cell walls of the foam and prevent such shrinkage or collapse of the foam. Various foam controlling agents can be used to control the cell size of the foams. The production of moldable foam beads is not disclosed.
Similarly, U.S. Pat. No. 3,644,230 discloses an improved foam extrusion process in which surfactants such as partial esters of fatty acids are used to reduce or prevent the collapse or shrinkage of closed-cell foam structures following extrusion. The production of moldable foam beads is not disclosed.
Improved methods of producing moldable beads of foamed thermoplastic polymers such as polyolefins, e.g., polyethylenes, are clearly needed; for example, methods which would not require pressure treatment or radiation. Furthermore, moldable beads which need not be pressurized or crushed prior to molding are needed to simplify various molding processes and to make them more economical.